Field of the Invention
The present invention relates to a numerical control apparatus for controlling a wire electric discharge machine which performs taper machining.
Description of the Related Art
FIG. 7 is a diagram describing taper machining performed by a wire electric discharge machine. When taper machining is performed using a wire electric discharge machine, a distance between upper and lower guides L is set before the start of machining. The distance between upper and lower guides L is a vertical distance between a lower wire guide 112 and upper wire guide 111. A taper deviation vector T of a UV axis is calculated from the distance between upper and lower guides L, a wire tilt angle command θ, and a wire tilt direction command θ and position of the UV axis is controlled such that the wire tilt angle and wire tilt direction conform to command values.
Japanese Patent Laid-Open No. 1-257525 discloses wire cut electric discharge machining method which reads position of the upper guide from an NC program when a Z axis moves in response to an automatic operation command during automatic operation, i.e., during taper machining, calculates height from a table surface to the upper guide based on the read data, and calculates taper data.
FIG. 8 is a diagram describing an issue arising if the Z axis is moved while interrupting or continuing taper machining. If the Z axis is moved while interrupting or continuing taper machining, a discrepancy occurs between the distance between upper and lower guides L set before the start of machining and actual distance between upper and lower guides. Consequently, the wire tilt angle does not conform to the command. Therefore, if the Z axis is moved while interrupting or continuing taper machining, it is not possible to just proceed with the taper machining.
Thus, for example, on a taper machining block, it is not possible to carry out cutoff machining using a core fall prevention jig such as a magnet, i.e., carry out cutoff machining by manually raising the Z axis and mounting the jig. Also, a workpiece whose plate thickness changes cannot be taper-machined by moving up and down the Z axis.
Japanese Patent Laid-Open No. 1-257525 does not disclose a method for controlling the position of the UV axis in such a way that the wire tilt angle does not change when the Z axis is moved manually during automatic operation, i.e., during taper machining. This is apparent from the following descriptions:    “It is determined whether the Z axis is moved automatically or manually (S1) if the Z axis is moved automatically and the T−U value (distance between the table surface and upper guide) of the electric discharge machine is stored in the memory of the NC (numerical calculation) device side (S2), taper data is calculated via T−U value calculation (S3).” (bottom line, upper left column, page 2);    “On the other hand, if the Z axis is moved manually in S1, the height of the upper guide is entered manually (S21). Subsequent actions are similar to conventional ones” (line 5, lower right column, page 2); and    The “T−U value calculation” and “taper data calculation” in S3 in FIG. 1 are performed only when it is determined in S1 that the Z axis is moved automatically.
Also, no method is disclosed for controlling the position of the UV axis in such a way that the wire tilt angle does not change when the Z axis is moved in response to an automatic operation command during automatic operation, i.e., during taper machining.